1. Field of the Invention
The present invention relates to the field of the acoustic pressure measurement, in particular in underwater environment.
2. Description of the Related Art
The acoustic sensors such as the hydrophones are conventionally used in an underwater environment to detect acoustic pressure variations that may be caused by seismic waves, by the presence of sea mammals or ships, for example.
They may be used in a static configuration and be deployed on the sea bottom to carry out an acoustic monitoring, or may be towed by a ship or a submarine.
Most of the known acoustic sensors are based on the use of piezoelectric components whose deformation causes a pressure variation that can be measured electronically.
However, such sensors require a local installation, which make them difficult to use for the towed applications, as a seismic sensor for example. Moreover, the piezoelectric components are sensitive to the electromagnetic disturbances.
To remedy these drawbacks, there exist optical fibre hydrophones of the DFB FL type (“Distributed Feedback Fibre Laser”) comprising a Bragg grating, and which have the property to emit very fine wavelengths sensitive to the mechanical stress applied to the optical fibre. Measuring these variations of emitted wavelengths makes it possible to deduce the stress applied to the optical fibre and thus the external pressure.
This type of acoustic sensor present advantages such as the absence of electronic components in the immerged part, which make them easier to tow, and the possibility to multiplex several sensors on a same fibre.
However, these optical fibre sensors have an insufficient sensitivity for the detection of low variations of pressure.
To amplify these low variations of pressure, it is known to mechanically amplify the mechanical stress applied to the optical fibre.
Optical fibre sensors comprise an optical fibre surrounded with a cylinder made of elastomeric resin, so as to increase the stresses at the optical fibre by applying shear stresses to the optical fibre generated by the elastic cylinder.
However, such technology does not make it possible to reach the required sensitivities for the detection of small variations of acoustic pressure. Moreover, the Bragg grating being encapsulated in the resin, its operation is modified and its laser frequency decreases.
The document WO 2006/034538 discloses an optical fibre fastened to a flexible support. The latter makes it possible to amplify the acoustic signals.
There also exist acoustic sensors of the “toothpaste tube” type, including an optical fibre provided with a Bragg grating surrounded with a deformable casing filled with an incompressible fluid. The laser fibre is integral with the ends of the deformable casing.
The drawbacks of these prior art hydrophones lie in that they provide too low sensitivities, and are too bulky.
When they are used as streamers towed by a ship or a submarine, and when the acoustic monitoring operation is terminated, the optical fibre is wound with the hydrophone(s) around a winch drum, which is integral with the ship or the submarine.
The prior art hydrophones having high dimensions, they require the use of great diameter drums and do not permit to obtain a homogeneous optical fibre winding.